Most power sources provide a current at a fixed voltage.
In some situations a current at a different or variable voltage level is desired. For example, the transmitter in a cellular device requires a wide output range, but the highest output power is rarely used. Therefore, to ensure that the transmitter can transmit at the highest output power, the power source delivers voltage to the power amplifier for the transmitter at the highest output power.
However, providing voltage continuously at a high level when it is not continuously required for the efficient functioning of a device is a drain on a limited power source, such as a battery for mobile devices.
One method of varying a fixed voltage is by using a switched mode power supply (SMPS).
A SMPS uses a duty cycle to determine the frequency of switching in a voltage converter to determine the voltage level at the output of the voltage converter.
There are several different types of voltage converters for SMPSes including buck converters which decrease voltage, boost converters which increase voltage and buck/boost converters which can either increase or decrease voltage.
In existing systems a controller uses a reference voltage and a control loop to determine the duty cycle. The control loop feeds an output of the voltage converter back to the controller.
However, the use of a control loop can cause difficulties. For example, the control loop may become unstable, when output voltage is at the maximum for buck converter, or, in case of buck-boost converter, when the output voltage is about equal to the battery voltage. These instabilities can create extra noise in the output voltage. This is disadvantageous when the output voltage is provided to a component which is vulnerable to noise such as the power amplifier for signal transmissions. In such cases the radio frequency transmitted signal can be reduced in quality due to the noise.
There is a desire for a SMPS which minimises noise generation in the output voltage.
It is an object of the present invention to provide a switched mode power supply which minimises noise generation in the output voltage, or at least provides a useful alternative.